1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device which includes nitride group semiconductor functional layers.
2. Description of the Related Art
A high electron mobility transistor (HEMT) is popular as an electronic device including gallium nitride (GaN) group compound semiconductors. The HEMT has excellent carrier mobility and high-frequency characteristics.
The HEMT is constituted in a nitride group semiconductor functional layer which includes a GaN layer functioning as a channel layer and an aluminum gallium nitride (AlGaN) layer laid over the GaN layer by the hetero junction and functioning as a barrier layer. A two-dimensional electron gas channel (2DEG) where high speed carriers move is produced near the hetero junction of the GaN layer. A source electrode and a drain electrode are connected to the 2DEG channel by ohmic contact. A gate electrode is placed between the source and drain electrodes. The foregoing HEMT can accomplish high carrier density because of a piezoelectric field generated in response to intrinsic polarization and lattice mismatching. Such an HEMT is disclosed in WO 03/071607 A1.
In the foregoing publication, the barrier layer between the drain and gate electrodes, and a barrier layer between the gate and source electrodes So have the same thickness. It is seems that the cited invention does not seem to pay any consideration to the following: to modulate the carrier density in the 2DEG gas channel or the electric field to a channel direction, i.e. a carrier moving direction, by varying the thickness of the barrier layers.
The present invention has been contemplated in order to overcome the foregoing problem, and provides a semiconductor device in which a thickness of a barrier layer between drain and gate electrodes and a thickness of a barrier layer between gate and source electrodes are varied in order to modulate carrier density and electric field toward a carrier moving direction in a two-dimensional carrier gas channel generated in a nitride group semiconductor functional layer. Since the two-dimensional carrier gas channel has an expanse, the term “carrier density” may be replaced by the term “a width of two-dimensional carrier gas channel”.